In a microphone in which acoustic energy from a sound source is converted into electric output through an oscillating element which oscillates in response to the acoustic energy transmitted thereto, there are two types of microphones which are mainly classified into a condenser microphone in which electric output is obtained on the basis of the relative displacement of an oscillator and an stator, and a dynamic microphone which electric output is obtained on the basis of the relative speed of an oscillator and an stator.
The relative displacement or speed of the oscillator and the stator is often affected by the vibration of the microphone. For the reason, there is a problem of the noise component contained in an normal converted electric signal from the acoustic energy. This is because that when a microphone casing of the microphone is displaced in one direction, the oscillator tends to sustain the mass at the first position. Therefore, noise component is generated.
Since the microphone is most used graspingly in the user's hand, the microphone case is susceptible to the occurrence of the vibration. Particularly to the dynamic microphone, a magnetic circuit is fixed to the microphone case, while a diaphragm is vibratingly supported on the microphone case. Therefore the dynamic microphone is susceptible to larger displacement of the diaphragm thereof relative to the magnetic circuit, when the vibration of the microphone case is generated.
It is generally known that a noise signal at highest level is generated by a directional microphone, and a nondirectional microphone and an nondirectional condenser microphone follows in order of the attenuating effect of suppression mode which includes mass, resistant and elastic control.
In noise component in relatively high frequencies of the noise component produced by the vibration of the microphone casing, there is not a specific directivity, since such the noise component propagates in a stationary manner to a transducer unit through a path which passes to a diaphragm across the microphone casing and a elastic support member. However, in noise component in relatively low frequencies of the noise component produced by the vibration of the microphone casing, there is a cos .theta. directivity.
In conventional known methods for reducing such the noise component, there are proposed as follows:
a method by which a vibration isolation is made by means of visco-elastic member such as rubber to be mounted on a microphone casing, which is so called shock mount method (e.g. Japanese Patent Laid-Open No. 197000/1989), and PA1 a method by which an oscillation-detecting unit for detecting only oscillation noise is provided in a microphone in addition to an transducer unit, in a manner that one output signal is modulated against other signal (e.g., U.S. Pat. No. 2,835,735).
The vibration isolation which the above-stated shock mount effects, depends on the resonance frequency and the sharpness of the resonance of the oscillation system Accordingly, it is expected that the use of the shock mount enables only the effect which reduces the noise component in the frequency band where is a frequency higher than a relational frequency against a resonance frequency. In order to permit an expansion in a frequency band where provides allowance for effective vibration isolation, it is considered that a resonance frequency is set at lower level.
However, when the resonance frequency is set at the lower level, the transducer unit undergoes constant displacement against the normal position due to vibration caused by a microphone casing, so that the transducer unit is suffered from the collision with the inner surface of the microphone casing and the generation of the larger vibration noise of the low frequency component.
In the method for modulating one signal against other signal, a transducer unit for absorbing sound wave and an oscillation-detecting unit having same conversion mode as the transducer unit are used, wherein some noise signal is allowed to decrease satisfactorily in a manner that an adjustment of the phase and level of the output signal between the transducer unit and the oscillation-detecting unit is given by substraction.
However, in order to provide for equal phase and level in the wide frequency band of the output signals from both the units, it is not only required to carefully adjust the frequencies in the extreme but also the adjustment is essentially too difficult. Accordingly, there are need for adequately placing a limit of a frequency band area where oscillation is allowed to decrease, and also using a shock mount which expects a decrease in oscillation in other frequencies.
Furthermore, the oscillation-detecting unit is provided in an enclosure which prevents a propagation of acoustic energy from a sound source. In the oscillation-detecting unit, the oscillation is detected in an enclosed space to cause the frequency to be increased to a level at which the diaphragm is allowed to resonate, whereby output signal level decreases.
In addition, the transducer unit is disposed in a free space, while the oscillation detector unit is disposed in an enclosed space. As a result, the transducer unit is provided under an environment different than one of the oscillation-detector unit. Therefore, the output signals from both of the units undergo variations in phase and level which are previously adjusted due to a rise in the temperature, whereby the adjustment of the phase and the level is not normally operated to cause an increase of noise component. For the reason, it is required to adjust both phases and levels of output signals from the transducer unit and the oscillation-detecting unit. However, the adjustment of phase and levels between the two output signals needs extremely difficult operation.